1. Field of the Invention
This invention relates to the electroless deposition of copper and provides a specific improvement over the invention disclosed in copending application Ser. No. 909,209, now U.S. Pat. No. 4,209,331, filed May 25, 1978 and assigned to the assignee of the present application. In particular, this invention relates to the electroless deposition of copper utilizing a non-formaldehyde type reducing agent to reduce copper ions dissolved in solution, in the presence of nickel or cobalt ions, to metallic copper to provide metal deposits or films of a desired thickness, greater than the limiting thickness obtainable before, on a suitably prepared substrate contacted by the solution as a continuous plating step. By "continuous plating" as used herein is meant a plating operation wherein the plating thickness increases with time at a substantially constant rate similar to the initial plating rate.
In the above-mentioned copending application Ser. No. 909,209, now U.S. Pat. No. 4,209,331, there is disclosed the invention that non-formaldehyde type reducing agents can be usefully employed in commercial installations as a reducer for copper ions in electroless plating baths by observing certain limitations to produce an electrically conductive metallic base or film on suitably prepared substrates, and particularly on catalyzed non-conductive substrates. One such reducing agent disclosed as being especially useful is hypophosphite. The present invention provides any desirable thickness of continuously plated metallic copper in such non-formaldehyde type reducing agent systems through the inclusion of nickel or cobalt ions as autocatalytic agents in the plating bath solutions.
2. Description of the Prior Art
The description of the prior art contained in copending application Ser. No. 909,209, now U.S. Pat. No. 4,209,331, referred to above, and which is incorporated by reference herein, reveals that conventional electroless plating as commercially practiced in the deposition of copper onto various substrates, especially non-conductive substrates, almost without exception uses highly alkaline formaldehyde solutions of divalent copper complexed with various well known agents such as Rochelle salt, amines and others. Given the teaching and experience of the prior art discussed therein, it was surprising and unexpected that a non-formaldehyde type reducing agent, such as hypophosphite, would successfully reduce copper ions to metallic copper for electroless deposition while also providing advantages not available in the typical formaldehyde systems.
While the technical literature clearly establishes that hypophosphite agents are effective and universally used as reducing agents in electroless nickel deposition techniques, there is no suggestion in the prior art that the hypophosphite of nickel baths can be substituted for formaldehyde in copper baths. Thus, in the prior patents, where both electroless nickel as well as copper baths are disclosed, the bath composition examples invariably employ formaldehyde type reducing agents for the copper formulations and, in contrast, hypophosphites for the nickel formulations.
A recent U.S. Patent, No. 4,036,651, teaches incorporation of sodium hypophosphite as a "plating rate adjuster" in an alkaline formaldehyde type electroless copper solution. The patent states expressly "Although sodium hypophosphite is, itself, a reducing agent in electroless nickel, cobalt, palladium and silver plating baths, it is not a satisfactory reducing agent (i.e., will not reduce Cu++--CU.degree.) when used alone in alkaline electroless copper plating baths." In discussing the disclosed baths, the patent states that the sodium hypophosphite is not used up in the plating reaction but instead appears to act as a catalyst for the formaldehyde reduction.
U.S. Pat. No. 3,716,462 states the production of a copper coating on a zinc or zinc alloy body may be obtained using an electroless plating solution consisting essentially of a soluble copper salt, e.g. copper sulfate, a complexing agent, e.g., citric acid, and a reducing agent, e.g. sodium hypophosphite. However, the patent states "heretofore it has been considered difficult and impractical to apply an electroless copper plating to zinc or its alloys", a view which is contrary to accepted common knowledge of plating base metal such as zinc or steel through immersion in a copper-containing solution. Moreover, the patent is limited to plating on zinc whereas "electroless deposition" is generally considered to refer to adhering a metal coating on a non-conductive substrate. Furthermore, it appears that the hypophosphite present in the solutions of the patent has no true utility in the plating process described.